Copy weights from other bone and invert them, how?
Ok, it was a pretty basic stuff after all.

In the following scenario: with vertex groups named after the bones, select the vertex bone of the upper arm, in my case "arm1.L", paint the weights. Now duplicate the vertex group. Rename the duplicated vertex group after the shoulder bone, in my case "shoulder.L". Now, in the left panel, below Weight Tools, there is a button that says "Invert".

If the "shoulder.L" group already exists, like in my case, the only way is to delete it first as I could not find a way to transfer the weights between groups other than duplicating a group.